y = 0
k = 1
a = 1
b = -2

#主函数
if __name__ == '__main__':
    print("开始")
    user_input = input("请输入x：")

    x = float(user_input)
    if(x<0):
        y = k * x + 3
    else:
        y = a * x * x + b * x +3
    print("输出y: "+str(y))
    print("结束")